Step 1: Gradient

Select the gradient tool from the toolbar and make sure the foreground colour is the colour that you want the graduated effect to be on the sky. I chose blue for this tutorial. Your gradient styles and how it will be applied to the image can be selected in the tool bar that runs horizontally under the menus.

Step 2: Draw on your sky

Now click at the top of the sky and, while holding down the mouse, draw a line so it reaches below the horizon.

Step 3: Blend mode

You'll see that the sky is now a little brighter but extends over your landscape slightly. To rectify this change the blend mode of the layer to Multiply and reduce the opacity slightly. 70% was fine for this image but you may need to reduce it by more.

If you don't like the colour you can change it by going to Image>Adjustment>Hue/Saturation. Simply play around with the sliders until you're happy with the changes.